It is known to apply various chemical treatments to fibers such as glass fibers and carbon fibers to facilitate handling of the fibers, improve processability for compounding and downstream processes such as injection molding, and to improve strength and other properties for specific end-use applications. A surface treatment, which may also be referred to as a coating composition, sizing composition, or sizing material, may be applied to at least a portion of a surface of a fiber to protect it from abrasion and to assist in processing. This process of applying the surface treatment is commonly referred to as sizing.
In one known process, a sizing material is applied to a target material, for example a plurality of fibers, using a solvent. First, the sizing material is dissolved in the solvent. Next, the solvent is applied to the fibers so that the sizing material is applied to at least a portion of a surface of the fibers. After the solvent is applied to the fibers, the fibers coated with sizing material are heated to evaporate the solvent.
Polyaryletherketones (PAEK) polymers are of interest to use as sizings for fibers used as reinforcements for plastic type materials—especially those processed at higher temperatures than can be tolerated by more common sizings. A PAEK polymer powder may be from the group consisting of polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherketone (PEK), polyetheretherketoneketone (PEEKK) or polyetherketoneetherketoneketone (PEKEKK).
U.S. Patent Application No. 2011/0294943 (the '943 application) discloses a method of applying amorphous (i.e. non-crystalline) PAEK as a sizing composition to fibers. The '943 application recognizes that many polymeric resins, especially highly crystalline engineering thermoplastics such as crystalline PAEK, do not exhibit good adhesion to various fiber surfaces. To overcome this problem, the '943 application proposes a method of coating a fiber with amorphous PEKK. The disclosure of the '943 application is incorporated herein in its entirety.
A disadvantage of the method disclosed in the '943 application is that it requires aggressive, toxic, and potentially explosive solvents to dissolve the amorphous PAEK. The need to use such dangerous solvents greatly inhibits broader adoption and use of sizing compositions comprising amorphous PAEK. The '943 application, for example, teaches dissolving a PAEK resin using methylene chloride or concentrated sulfuric acid to create a solution for sizing a fiber. Such wetting and doctoring processes are well known and, therefore, are not detailed herein except to note that they require highly corrosive solvents that are not commonly used for reasons of safety, environment and machinery lifetime. Due to the low solubility of PAEK resins in any solvent system, this process must of necessity create copious volumes of difficult (toxic, acidic) to manage waste. As a result, such processes are expensive and dangerous, limiting their usefulness and discouraging adoption by industry.
What is desired, therefore, is to provide a system and method of applying a sizing comprising a PAEK polymer to a fiber in which a less volatile solvent, as compared to known methods, is used.